Auto merge of #54596 - mjbshaw:drop, r=RalfJung

Make core::mem::needs_drop a const fn

This fixes #51929.

src/test/run-pass/const-needs_drop.rs

@@ -0,0 +1,39 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(const_needs_drop)]
+
+use std::mem;
+
+struct Trivial(u8, f32);
+
+struct NonTrivial(u8, String);
+
+const CONST_U8: bool = mem::needs_drop::<u8>();
+const CONST_STRING: bool = mem::needs_drop::<String>();
+const CONST_TRIVIAL: bool = mem::needs_drop::<Trivial>();
+const CONST_NON_TRIVIAL: bool = mem::needs_drop::<NonTrivial>();
+
+static STATIC_U8: bool = mem::needs_drop::<u8>();
+static STATIC_STRING: bool = mem::needs_drop::<String>();
+static STATIC_TRIVIAL: bool = mem::needs_drop::<Trivial>();
+static STATIC_NON_TRIVIAL: bool = mem::needs_drop::<NonTrivial>();
+
+fn main() {
+    assert!(!CONST_U8);
+    assert!(CONST_STRING);
+    assert!(!CONST_TRIVIAL);
+    assert!(CONST_NON_TRIVIAL);
+
+    assert!(!STATIC_U8);
+    assert!(STATIC_STRING);
+    assert!(!STATIC_TRIVIAL);
+    assert!(STATIC_NON_TRIVIAL);
+}

src/test/run-pass/union/union-nodrop.rs

@@ -57,15 +57,13 @@ impl<T> Drop for ActuallyDrop<T> {
 }
 
 fn main() {
-    unsafe {
-        // NoDrop should not make needs_drop true
-        assert!(!needs_drop::<Foo>());
-        assert!(!needs_drop::<NoDrop<u8>>());
-        assert!(!needs_drop::<NoDrop<Box<u8>>>());
-        // presence of other drop types should still work
-        assert!(needs_drop::<Baz>());
-        // drop impl on union itself should work
-        assert!(needs_drop::<ActuallyDrop<u8>>());
-        assert!(needs_drop::<ActuallyDrop<Box<u8>>>());
-    }
+    // NoDrop should not make needs_drop true
+    assert!(!needs_drop::<Foo>());
+    assert!(!needs_drop::<NoDrop<u8>>());
+    assert!(!needs_drop::<NoDrop<Box<u8>>>());
+    // presence of other drop types should still work
+    assert!(needs_drop::<Baz>());
+    // drop impl on union itself should work
+    assert!(needs_drop::<ActuallyDrop<u8>>());
+    assert!(needs_drop::<ActuallyDrop<Box<u8>>>());
 }